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The coil is oriented coaxially inside the gap, a small circular volume (a hole, slot, or groove) in the magnetic structure within which it can move back and forth. The gap establishes a concentrated magnetic field between the two poles of a permanent magnet; the outside of the gap being one pole and the center post (a.k.a., the pole-piece) being the other. The center post and back-plate are sometimes a single piece called the yoke.

A subwoofer is a woofer driver used only for the lowest part of the audio spectrum: typically below 100-120 Hz. Because the intended range of frequencies in these is limited, subwoofer system design is usually simpler in many respects than for conventional loudspeakers, often consisting of a single subwoofer driver enclosed in a suitable cabinet or enclosure.